Electromagnetic shielding material having carbon nanotube and metal as eletrical conductor

ABSTRACT

Disclosed is an electromagnetic shielding material with enhanced shielding effectiveness and mechanical property by employing a carbon nanotube and a metal as an electrical conductor. The electromagnetic shielding material includes a polymer resin for a matrix and two conductive fillers having a carbon nanotube and a metal, wherein a volume percent of the carbon nanotube ranges about 0.2% to about 10% and a volume percent of the metal powder ranges about 7.0% to about 30% so that the total volume percent of the conductive filler is in a range of about 7.2% to about 40%.

TECHNICAL FIELD

The present invention relates to an electromagnetic shielding material;more particularly to an electromagnetic shielding material with enhancedelectromagnetic shielding effectiveness by employing a carbon nanotubeand a metal powder therein as an electrical conductor.

BACKGROUND ART

Electromagnetic waves can cause malfunctions in some electronic devices.More importantly negative influence on the health of human beings isalso suspected. Therefore, most of countries regulate an electromagneticinterference and resistance according to an international standard,i.e., an International Special Committee on Radio Interference (CISPR)of an International Electrotechnical Commission (IEC) and anElectromagnetic Compatibility (EMC). Therefore, in order to sell variouselectronic devices, the electronic devices should meet the regulationfor the electromagnetic waves.

To protect the electronic devices and rooms interiors from theelectromagnetic waves, an electromagnetic shielding material is mainlyused. The electromagnetic material for the aforementioned purpose shouldpossess good electric conductivity to minimize the penetration of theelectromagnetic waves into the material and low magnetic permeability toconvert magnetic energy into heat.

The prior art electromagnetic shielding material is typicallymanufactured by dispersing a metal powder or a carbon nanotube with highelectrical conductivity into a polymer such as silicon rubber,polyurethane, polycarbonate and epoxy resin, wherein a volume percent ofthe metal powder or the carbon nanotube is higher than 30%. Herein, themetal powder uses mainly a silver powder or a silver-coated copperpowder which has high electrical conductivity. When the silver powderover 30 volume % is dispersed into the polymer, it is possible to obtainthe electromagnetic shielding material having a volume resistivity lessthan 0.01 ohm-cm and an electromagnetic shielding effectiveness of about50 dB.

In recent years, however, there is still required for theelectromagnetic shielding material having much more enhanced volumeresistivity and electromagnetic shielding effectiveness, to meet therigid regulation of the electromagnetic shielding interference. To solvethe above problem, lots of metal powders such as silver powders aredispersed into the polymer. However, as metal powders are dispersed moreand more into the polymer, mechanical property such as impact strengthbecomes deteriorated while the electromagnetic shielding effectivenessbecomes enhanced. Thus, it is difficult to manufacture theelectromagnetic shielding material stably when large amount of the metalpowder is dispersed in the polymer.

Meanwhile, since the carbon nanotube has been developed by S. Iijima,disclosed in Nature Vol. 354, page 56, published in 1991, variousresearches for the carbon nanotube has been advanced up to date. Thecarbon nanotube has advantageous merits as followings in comparison withthe other conventional materials; it has high elastic coefficientranging from about 1.0 Mpa to about 1.8 Mpa; it has an enhanced heatresistant property to endure at 2,800° C. in vacuum state; its heatconductivity is two times to that of a diamond; and its currenttransferring capability is about 1,000 times to that of copper.Therefore, the carbon nanotube is widely applied to a nano-scaledelectric/electronic device, a nano-sensor, a photoelectric device and ahigh-functional composite. In case of applying the carbon nanotube tothe electromagnetic shielding, it is possible to obtain theelectromagnetic shielding material with low volume resistivity like asemiconductor in case of dispersing the carbon nanotube beyond 0.04volume % into the polymer, because small amount of the carbon nanotuberenders a conductive network formed in the polymer.

However, the prior art electromagnetic shielding material has ashortcoming that it is difficult to obtain a desired electromagneticshielding effectiveness. That is, though there are much more carbonnanotube dispersed into the polymer, the electromagnetic shieldingmaterial shows relatively high volume resistivity, i.e., about 10 ohm-mand poor shielding effectiveness. Moreover, it is hard to disperse evensmall amount of the carbon nanotube into the polymer so that there is alimitation to apply it to the electromagnetic shielding material.

DISCLOSURE OF INVENTION

It is, therefore, an object of the present invention to provide anelectromagnetic shielding material with an enhanced property byemploying a carbon nanotube and a metal powder as an electricalconductor.

In accordance with the present invention, there is provided anelectromagnetic shielding material comprising: a polymer resin for amatrix; and a conductive filler including a carbon nanotube and a metal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and aspects of the invention will become apparent from thefollowing description of the embodiments with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic conceptual scheme setting forth composition of anelectromagnetic shielding material in accordance with a preferredembodiment of the present invention; and

FIG. 2 is a perspective view illustrating an apparatus for measuringsurface resistivity of the electromagnetic shielding material inaccordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an electromagnetic shielding material in accordance with apreferred embodiment of the present invention will be described indetail with reference to the accompanying drawings.

FIG. 1 shows a composite of the electromagnetic shielding material inaccordance with the present invention.

Referring to FIG. 1, the inventive electromagnetic shielding materialincludes a matrix of a polymer resin 103 and a conductive filler havinga metal powder 102 and a carbon nanotube 101. Herein, a volume percentof the carbon nanotube ranges from about 0.2% to about 10% and the metalpowder is dispersed in the polymer resin 103 with a volume percent in arange of about 7.0% to about 30%. In this case, it is noted that a totalvolume percent of the conductive filler should be in the range of about7.2% to about 40%.

To begin with, polymer which can compound the carbon nanotube 101 andthe metal powder 102 can be used for the polymer resin 103 regardless ofits molecular weight, density, molecular structure and functional group.For instance, the polymer resin 103 may employ a general-purpose polymersuch as silicon rubber, polyurethane, polycarbonate, polyacetate,polymethyl methacrylate, polyvinyl alcohol,Acrylonitrile-Butadiene-Styrene terpolymer (ABS) or the like. Inaddition, functional thermosetting resin such as epoxy, polyimide or thelike may also be utilized for the polymer resin 103. Furthermore,polymer material obtained through blending of the aforementionedpolymers can also be used for the polymer resin 103. However, in case ofrequiring high thermal resistance and high resistance against mechanicalimpact strength, silicon rubber or polyurethane is more adequate for thepolymer resin 103.

In general, the carbon nanotube 101 is manufactured by a predeterminedmethod selected from the group consisting of a chemical vapordeposition, an arc discharge, a plasma torch and an ion impact, whereinthe carbon nanotube has a single-walled nanotube (SWNT) or amulti-walled nanotube (MWNT). Typically, in order to enhance adispersion property of the carbon nanotube 101 in the polymer resin 103,a predetermined process should be carried out such as a refining processby aid or a reforming process by fluorine gas. But, in the presentinvention, the carbon nanotube 101 can be utilized for the conductivefiller without carrying out the refining process or the reformingprocess.

From the result of a Fourier Transform Infrared (FT-IR) spectroscopy, itis understood that the carbon nanotube 101 of the present invention ispreferably selected from the group consisting of a nanotube having aphenyl-carbonyl C—C stretch bonding peak existing between about 1,300cm⁻¹ and about 1,100 cm⁻¹, a nanotube having a phenyl-carbonyl C—Cstretch bonding peak existing between about 1,300 cm⁻¹ and about 1,100cm⁻¹, a carbonic C—C stretch bonding peak existing between about 1,570cm⁻¹ and about 1,430 cm⁻¹ and a carboxylic C═O stretch vibration peakexisting at about 1,650 cm⁻¹, a nanotube having a phenyl-carbonyl C—Cstretch bonding peak existing between about 1,300 cm⁻¹ and about 1,100cm⁻¹, a carboxyl C═O stretch vibration peak existing at about 1,650 cm⁻¹and an —OH bonding peak existing at about 3,550 cm⁻¹, a nanotube havinga C—F bonding peak existing at about 1,250 cm⁻¹ and a combinationthereof. In case of not using the carbon nanotube aforementioned, thedispersion property and the electrical conductivity of the carbonnanotube may be deteriorated so as to have a bad effect on anelectromagnetic shielding effectiveness. Furthermore, in this case, thecarbon nanotube has a bad interaction between the metal powders 102 sothat the electromagnetic shielding effectiveness may become poor.

In the present invention, the carbon nanotube 101 has at least 0.2volume % with respect to the whole composite. Preferably, the volumepercent of the carbon nanotube should be in the range of about 0.2% toabout 10%. Provided that the volume percent of the carbon nanotube islower than about 0.2%, the interaction between the carbon nanotube 101and the powder metal 102 is too bad so that the electromagneticshielding material does not have a desired electrical conductivity. Onthe other hand, if the volume percent of the carbon nanotube 101 ishigher than about 10%, the electrical conductivity can be increased butthe dispersion property may be worsened, whereby it has a difficulty formilling and forming the electromagnetic shielding material.

The metal powder 102 for use in the present invention may employ aconventional metal powder such as a silver powder, silver-coated copperor the like. However, a predetermined conductive fiber can be usedinstead of the metal powder 102, e.g., a steel fiber, a copper fiber, analuminum fiber, a nickel fiber and so forth. Herein, the metal powder102 has such a characteristic that the electrical conductivity should behigher than about 10⁵ S/cm and the volume percent should be higher thanabout 7.0% If the volume percent of the metal powder 102 is lower thanabout 7.0%, the electrical conductivity may be decreased and theelectromagnetic shielding effectiveness may be deteriorated becausethere is not enough percolation by means of the interaction between themetal powder 102 and the carbon nanotube 101. Therefore, in the presentinvention, the volume percent of the metal powder 102 should bepreferably in the range of about 7.0% to about 30% in consideration ofhigh mechanical shock resistance of the polymer resin 103, a density ofthe electromagnetic shielding material and formability.

The inventive electromagnetic shielding material may further include apositive ionic dispersing agent, a negative ionic dispersing agent or anon-ionic dispersing agent for achieving an enhanced dispersionproperty. In addition, if necessary, there is introduced asilane-coupling agent such as an epoxy, carboxyl, amine or the like anda compatibilizer which may be required in case of blending at least twokinds of polymer resin.

A method for manufacturing the electromagnetic shielding material inaccordance with the present invention will be described hereinafter.

The method for manufacturing the inventive electromagnetic shieldingmaterial begins with preparing a raw material, i.e., the carbon nanotube101, the metal powder 102 and the polymer resin 103. Thereafter, the rawmaterial is mixed in a mixer such as a Henshel mixer, a Lodige mixer, ahomgenizer or the like. Subsequently, a mixture of the carbon nanotube101, the metal powder 102 and the polymer epoxy 103 are melted andkneaded by means of a two roll mill or a kneader and then the kneadedmixture is cooled to thereby obtain the electromagnetic shieldingmaterial. Herein, to facilitate the mixing process, a solvent which candissolve the polymer resin 103 may be utilized such as methyl ethylketone, alcohol, isoprophyl alcohol, toluene or a combination thereof.It is noted that the solvent should not exist in a final product, i.e.,the electromagnetic shielding material. Thus, the solvent remaining inthe product should be removed during a predetermined manufacturingprocess. By using the solvent, it is possible to manufacture theelectromagnetic shielding material with various shapes such as a bulktype, a coating type and a spray type.

An experimental result and an example for the present invention will beset forth in detail with reference to the accompanying tables 1 to 3.

In table 1, there is shown a volume resistivity and an electromagneticshielding efficiency measured in the experiment. Specimens used in theexperiment are provided by following steps. To begin with, afterweighting each material described in table 1, polyurethane is dispersedin methyl ethyl ketone solvent, wherein a volume ratio of thepolyurethane and the methyl ethyl ketone is 1 to 6. Subsequently, apredetermined amount of the carbon nanotube is stirred for about 30minutes with nitric acid diluted with a mixing ratio of 1 to 3 and anaqueous solution with 25 wt % of sulfuric aid and then add aqueoussolution is removed. The carbon nanotube and a silver powder are addedinto a solution mixed with polyurethane and methyl ethyl ketone.Thereafter, a solution mixed with the polyurethane, the carbon nanotube,the silver powder and the methyl ethyl ketone is stirred and isultrasonically treated. Then, after the methyl ethyl ketone solvent isremoved, the two roll mill process is carried out to thereby obtain aconductive composite. TABLE 1 Specimen Specimen Specimen Specimen #1 #2#3 #4 Polyurethane (Vol. %) 92.8 91.8 89.8 89.4 Carbon nanotube 0.2 0.20.2 0.6 (Vol. %) Silver powder (Vol. %) 7.0 8.0 10.0 10.0 Volumeresistivity 385.8 165.5 80.3 36.2 (10⁻³ ohm-cm) Electromagnetic 39.449.0 55.3 63.4 shielding effectiveness (dB)

Herein, the volume resistivity is measured by means of measuringapparatus, e.g., MIL-G-835288™ which is depicted in FIG. 2 and anohmmeter for measuring low resistance, e.g., 3,450 mΩ Hi-tester™. Thedetail description for the MIL-G-835288™ will be illustrated later. Eachspecimen #1 to #4 is provided with a shape having a rectangularparallelepiped of 25.4 mm×35.0 mm×0.5 mm. To find out volumeresistivity, first of all, a surface resistivity (Ω/square) is measuredand then the surface resistivity is converted to the volume resistivity(ohm-m).

Referring to FIG. 2, there is shown a perspective view setting forth theMIL-G-835288™ apparatus for measuring the surface resistivity.

In FIG. 2, input/output terminals of an external measuring apparatus 201are connected with a 4-terminal measuring probe having an insulator 202and a silver plated brass 203. Beneath the probe, there is disposed agasket sample 204, i.e., a specimen. Herein, it is possible to measurethe surface resistivity in case that the specimen has a predeterminedsize over than 1.4 inches×1 inch. The measured data is displayed at aliquid crystal display (LCD). Finally, the surface resistivity ismultiplied by a thickness of the specimen to thereby evaluate the volumeresistivity.

Meanwhile, the electromagnetic shielding effectiveness can be measuredwithin the range of about 50 MHz to about 6 GHz by using a networkanalyzer, e.g., Agilent 8722ESTM, axxording to an ASTM D4935-89specification and a 2-port flanged coaxial holder, wherein the specimenused in the experiment has a diameter of about 133.0 mm. The shieldingeffectiveness is evaluated by using S21 parameter after measuring ans-parameter and a return loss is evaluated by using S11 parameter.

Herein, S21 parameter means a transmission amount of an incidentelectromagnetic wave and is used for estimating the electromagneticshielding effectiveness.

Referring to table 1, in the electromagnetic shielding material of eachspecimen, the volume percent of the carbon nanotube is increased from0.2% to 0.6% and that of the silver powder is increased from 7.0% to10%. From table 1, it is understood that the volume resistivity isdecreased and the electromagnetic shielding effectiveness is increasedif the volume percent of the carbon nanotube is higher than about 0.2%and that of the silver powder is higher than about 7.0%.

Table 2 shows another experimental data of the volume resistivity andthe shielding effectiveness for each specimen in case of dispersing thecarbon nanotube in methyl ethyl ketone solution without any acidtreatment in accordance with the present invention. In table 2, since apreparation of each specimen is same to the preparation step describedin the experiment of table 1, further description for the specimen willbe omitted herein. TABLE 2 Specimen Specimen Specimen Specimen #5 #6 #7#8 Polyurethane (Vol. %) 89.8 89.4 89.0 78.0 Carbon nanotube 0.2 0.6 1.02.0 (Vol. %) Silver-coated 10.0 10.0 10.0 20. copperpowder (Vol. %)Volume rsistivity 120.7 75.7 35.4 3.7 (10⁻³ ohm-cm) Electromagnetic 51.255.9 63.7 86.5 shielding efficiency (dB)

In comparison with the experimental result of table 1, the experiment oftable 2 employs the silver-coated copper powder instead of the silverpowder and the other experimental conditions are similar to theexperiment of table 1. From tables 1 and 2, the inventiveelectromagnetic shielding material having a mixture of the carbonnanotube and the metal powder represents low volume resistivity and highshielding effectiveness in case of employing the carbon nanotube and themetal powder beyond 0.2 volume % and 7.0 volume %, respectively.

Referring to table 3, there are further another experimental data forspecimens having one conductive filler or two conductive fillers inwhich each volume percent of the carbon nanotube and the metal powder isout of the range of the inventive electromagnetic shielding material.TABLE 3 Speci- Speci- men men Specimen Specimen Specimen #9 #10 #11 #12#13 Silicon rubber 98.0 92.5 85.0 93.9 70.0 Carbon 2.0 7.5 15.0 0.1 0nanotube Silver powder 0 0 0 6.0 30.0 Vol. resistivity 58.3 0.783 Cannotbe 32.2 0.0365 (10⁻³ ohm-cm) fabricated Electromagnetic 0 5 0 63shielding efficiency (dB)

From table 3, if the electromagnetic shielding material Institutes withthe silicon rubber and the carbon nanotube like the specimens #9 and#11, the volume resistivity is very high and the shielding effectivenessis very low. Additionally, even though the electromagnetic shieldingmaterial has two conductive fillers of the carbon nanotube and thesilver powder, it shows poor volume resistivity and shieldingeffectiveness in case that the volume percent of the carbon nanotube andthat of the silver powder are below 0.2% and beyond 30.0%, respectively.In particular, if the electromagnetic shielding material is manufacturedby using the carbon nanotube only, the experimental data shows that theshielding effectiveness is extremely low. Moreover, provided that thecarbon nanotube exceeds to about 10 volume %, e.g., 15 volume % like thespecimen #11, the dispersion property is too deteriorated so that it isimpossible to fabricate the electromagnetic shielding material. Eventhough the electromagnetic shielding material shows good shieldingeffectiveness in case of filling the silver powder more than 30% in thesilicon rubber, it is difficult to obtain the electromagnetic shieldingmaterial with high mechanical shock resistance, light-weight materialand cost-effective material comparing with the present invention.

As described already, since the present invention employs two conductivefillers concurrently, i.e., the carbon nanotube and the metal powder, itis possible to obtain the electromagnetic shielding material with anenhanced shielding effectiveness and a physical/mechanical property.That is, the metal powder 102 in the polymer resin 103 is relativelysmall in comparison with the prior art electromagnetic shieldingmaterial so that the large amount of the polymer resin 103 provides goodmechanical property, i.e., high resistance against the mechanical shock.In addition, the present invention provides an advantageous merit thatsmall amount of the metal powder makes a manufacturing cost reduced andthe weight of the shielding material lightened. Furthermore, theinventive electromagnetic shielding material has another advantage thatit has an enhanced property for heat radiation because the carbonnanotube and the metal powder have good electrical conductivity.

The present application contains subject matter related to the Koreanpatent application No. KR 2003-72074, filled in the Korean Patent Officeon Oct. 16, 2003, the entire contents of which being incorporated hereinby reference.

Although the preferred embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

1. An electromagnetic shielding material comprising: a polymer resin for a matrix; and a conductive filler including a carbon nanotube and a metal.
 2. The electromagnetic shielding material as recited in claim 1, wherein a volume percent of the carbon nanotube ranges from about 0.2% to about 10% and a volume percent of the metal powder ranges from about 7.0% to about 30% so that the total volume percent of the conductive filler is in a range of about 7.2% to about 40%.
 3. The electromagnetic shielding material as recited in claim 1, wherein the carbon nanotube employs a single-walled carbon nanotube or a multi-walled carbon nanotube.
 4. The electromagnetic shielding material as recited in claim 3, wherein the carbon nanotube is manufactured by a method selected from the group consisting of a chemical vapor deposition, an arc discharge, a plasma torch and an ion impact.
 5. The electromagnetic shielding material as recited in claim 3, wherein the carbon nanotube is material selected from the group consisting of a nanotube having a phenyl-carbonyl C—C stretch bonding peak existing between about 1,300 cm⁻¹ and about 1,100 cm⁻¹, a nanotube having a phenyl-carbonyl C—C stretch bonding peak existing between about 1,300 cm⁻¹ and about 1,100 cm⁻¹, a carbonic C—C stretch bonding peak existing between about 1,570 cm⁻¹ and about 1,430 cm⁻¹ and a carboxylic C═O stretch vibration peak existing at about 1,650 cm⁻¹, a nanotube having a phenyl-carbonyl C—C stretch bonding peak existing between about 1,300 cm⁻¹ and about 1,100 cm⁻¹, a carboxyl C═O stretch vibration peak existing at about 1,650 cm⁻¹ and an —OH bonding peak existing at about 3,550 cm⁻¹, a nanotube having a C—F bonding peak existing at about 1,250 cm⁻¹ and a combination thereof.
 6. The electromagnetic shielding material as recited in claim 1, wherein the polymer resin is a general-purpose polymer selected from the group consisting of a silicon rubber, a polyurethane, a polycarbonate, a polymethyl methacrylate, polyvinyl alcohol, Acrylonitrile-Butadiene-Styrene terpolymer (ABS) and a combination thereof.
 7. The electromagnetic shielding material as recited in claim 1, wherein the polymer resin is a thermosetting resin selected from the group consisting of epoxy, polyimide and a combination thereof.
 8. The electromagnetic shielding material as recited in claim 1, wherein electrical conductivity of the metal is higher than 10⁵ S/cm.
 9. The electromagnetic shielding material as recited in claim 8, wherein the metal is a material selected from the group consisting of a silver powder, a silver-coated copper powder, a steel fiber, a copper fiber, an aluminum fiber and a nickel fiber. 